Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants, while it may be more difficult to tune inorganic emissive materials. As used herein, the term “organic material” includes polymers as wells as small molecule organic materials that may be used to fabricate organic opto-electronic devices.
OLEDs makes use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly popular technology for applications such as flat panel displays, illumination, and backlighting. OLED configurations include double heterostructure, single heterostructure, and single layer, and a wide variety of organic materials may be used to fabricate OLEDs. Several OLED materials and configurations are described in U.S. Pat. No. 5,707,745, which is incorporated herein by reference in its entirety.
One or more transparent electrodes may be useful in an organic opto-electronic device. For example, OLED devices are generally intended to emit light through at least one of the electrodes. For OLEDs from which the light emission is only out of the bottom of the device, that is, only through the substrate side of the device, a transparent anode material, such as indium tin oxide (ITO), may be used as the bottom electrode. Since the top electrode of such a device does not need to be transparent, such a top electrode, which is typically a cathode, may be comprised of a thick and reflective metal layer having a high electrical conductivity. In contrast, for transparent or top-emitting OLEDs, a transparent cathode such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 may be used. As distinct from a bottom-emitting OLED, a top-emitting OLED is one which may have an opaque and/or reflective substrate, such that light is produced only out of the top of the device and not through the substrate. In addition, a fully transparent OLED that may emit from both the top and the bottom.
The transparent cathodes as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 typically comprise a thin layer of metal such as Mg:Ag with a thickness, for example, that is less than about 100 angstroms. The Mg:Ag layer is coated with a transparent, electrically-conductive, sputter-deposited, ITO layer. Such cathodes may be referred to as compound cathodes or as TOLED (“Transparent-OLED”) cathodes. The thickness of the Mg:Ag and ITO layers in such compound cathodes may each be adjusted to produce the desired combination of both high optical transmission and high electrical conductivity.
The organic materials of an opto-electronic device may be very sensitive, and may be damaged by conventional semiconductor processing. For example, any exposure to high temperature or chemical processing may damage the organic layers and adversely affect device reliability. In addition, exposure to air or moisture may also damage the sensitive organic layers. Such exposure may also damage any Mg:Ag layer that may be present, because Mg:Ag is highly reactive. The organic materials may also be damaged by the processes used to deposit transparent electrode materials. While conventional processes and structures allow for the fabrication of operational organic devices with transparent electrodes, the yield with such processes may be less than optimal. Indeed, the yield may not be great enough to fabricate commercially viable displays, for example, where the failure of only a few percent of the devices may render the display unsuitable for commercial use. There is therefore a need for a method of fabrication and/or transparent electrode structures that result in higher yields.